1. Field of the Invention
The present invention generally relates to communication systems and, more particularly, to Orthogonal Frequency Division Multiplexing (OFDM) communication systems that utilize Low-Density Parity-Check codes.
2. Background of the Invention
Orthogonal Frequency Division Multiplexing (OFDM) is a digital transmission technique in which a signal is split into several narrowband channels at different frequencies. When modulating and demodulating signals, OFDM gives priority to minimizing the interference among the channels and symbols of the data stream. In this regard, some sort of Forward Error Correction (FEC) typically is used in conjunction with OFDM. In telecommunications, for example, FEC refers to a system of error control for data transmission where the receiving device has the capability to detect and correct fewer than a predetermined number or fraction of bits or symbols corrupted by transmission errors. FEC is implemented by adding redundancy to the transmitted information using some sort of coding or algorithm.
One type of OFDM receiver that employs FEC incorporates Low-Density Parity-Check (LDCP) codes. FIG. 1 is a block diagram that depicts a conventional OFDM communication system 100 utilizing LDPC as a means of FEC. An LDPC code is an error correcting code that provides a more reliable method of transmitting a message over a noisy transmission channel. LDPC uses a sparse parity-check matrix that is randomly generated and subject to sparsity constraints.
The system 100 can include a transmitter 105 which sends wireless signals via a channel 110 to a receiver 115. The transmitter 105 includes an LDPC encoder 120 which encodes bits of information. The encoded information bits are interleaved and mapped to Quadrature Amplitude Modulation (QAM) symbols in module 125. As known, interleaving generally scrambles the sequential order of the data stream according to a known pattern. The data stream can be interleaved with respect to time, frequency, or both time and frequency.
The resulting QAM symbols generated in module 125 are processed using an Inverse Fast Fourier Transform (IFFT) in module 130 to generate an OFDM symbol. Module 130 further adds a cyclic prefix to each OFDM symbol. The resulting signal can be transmitted via channel 110, i.e., as a wireless signal.
The receiver 115 includes a timing module 135 which selects samples to be processed using a Fast Fourier Transform (FFT) in module 140. The resulting signal is demodulated in demodulator 145. Functions including, but not limited to, channel estimation, equalization, automatic frequency control (AFC), and bit-log-likelihood ratio (LLR) generation also can be performed in demodulator 145. The bit-LLRs are de-interleaved, or descrambled, in de-interleave module 150. The de-interleave module 150 effectively reverses the interleaving process performed by module 125 to recover the proper data order. The resulting signal is provided to the LDPC decoder 155 where information bits are recovered.
OFDM communication system 100 only makes use of pilot symbols for the channel estimation process. As a result, when the channel is noisy, the channel error performance of communication system 100 with respect to data transmissions is sub-optimal notwithstanding the use of LPDC codes. Therefore, a need exists for a method and apparatus that provides for improved reliability for transmission of data over a noisy transmission channel in an OFDM communication system.